Ethylbenzene dehydrogenation is a strong endothermic, molecular increasing reversible reaction. Water vapor is often used as a diluent to reduce the partial pressure of ethylbenzene and to move the reaction toward the product. Water vapor in the reaction has the following effects:
(1) heating the reaction raw materials to the required temperature, avoiding heating ethylbenzene directly to a higher temperature and inhibiting the side reactions;
(2) adding heat, so as to avoid cooling due to endothermic reaction
(3) continuously removing coke deposit from the catalyst through the water-gas reaction, so that the catalyst is regenerated automatically; and
(4) maintaining the stability of Fe3+ in the active phase of catalyst, preventing excessive reduction thereof, and maintaining the stability of the catalyst.
However, the amount of water vapor added is subject to two factors: the allowable pressure drop and energy consumption of the reaction system. Advanced ethylbenzene dehydrogenation processes always pursue a higher yield of styrene at a lower steam-to-oil ratio (mass ratio of water vapor to ethylbenzene in the feed). Operation under low steam-to-oil ratio is one of the important measures for a styrene plant to reduce energy consumption.
The catalyst for dehydrogenation of ethylbenzene to styrene is an iron-based catalyst wherein iron oxide is the main active component and potassium oxide is the main promoter. Potassium can increase the activity of iron oxide by an order of magnitude, and can promote the water-gas reaction to remove carbon deposit and make the catalyst regenerate automatically. However, potassium easily migrates and gets lost during the reaction, which is an important cause for the deactivation of the catalyst. If an average catalyst is used in the dehydrogenation of ethylbenzene under the condition of low steam-to-oil ratio (water vapor/ethylbenzene), the water-gas reaction rate will decrease and the carbon deposition on the catalyst surface will increase, resulting in poor stability. In addition, during ethylbenzene dehydrogenation at low steam-to-oil ratio, as the partial pressure of hydrogen in the reaction system increases and the reducibility increases, part of the Fe3+ in the catalyst will be reduced, which will cause the crystal structure of the catalyst to change, resulting in a decrease of the strength of the catalyst. However, large ethylbenzene dehydrogenation units are operated at low steam-to-oil ratio in order to save energy. At the same time, due to their high catalyst loading quantity, large units have higher requirement for the catalyst strength in order to prevent the catalyst from being crushed. Researchers have made many attempts to develop a low steam-to-oil ratio catalyst.
European Patent No. 0177832 reports that, with addition of 1.8-5.4 wt % magnesia, the catalyst demonstrates high and stable performance at steam-to-oil ratios below 2.0 wt. %. However, the catalyst has higher potassium content.
U.S. Pat. No. 4,535,067 discloses that a portion of the potassium in the catalyst is added in the form of potassium nepheline doublets, but the catalyst has a conversion rate less than 65%, selectivity no more than 93% and styrene yield less than 60%, which is relatively low. Moreover, there is no reference to the life of the catalyst.
At present, in most of the disclosed low steam-to-oil ratio ethylbenzene dehydrogenation catalysts, researchers often add in a binder to improve catalyst strength in order that the catalyst could be operated under low steam-to-oil ratio conditions.
Patent CN103768150 discloses a low steam-to-oil ratio ethylbenzene dehydrogenation catalyst and it's preparing method, wherein vanadium is introduced into an iron-potassium-cerium-tungsten-magnesium system, and 2 to 5% binder selected from kaolin, diatomaceous earth or cement is also added.
Patent CN101279266A discloses an energy saving ethylbenzene dehydrogenation catalyst, wherein nickel oxide and another light rare earth oxide are added into a Fe—K—Ce—W—Mg system, and a binder selected from kaolin, diatomaceous earth or cement is also added in order to maintain the strength of the catalyst.
Patent CN101279268A discloses an energy saving ethylbenzene dehydrogenation catalyst, wherein boron trioxide and niobium oxide are added into a Fe—K—Ce—W system, and a binder selected from kaolin, diatomaceous earth or cement is also added in order to maintain the strength of the catalyst
Patent CN10127926A discloses a catalyst for dehydrogenation of ethylbenzene at low steam-to-oil ratio, wherein bismuth oxide and beryllium oxide are added into a Fe—K—Ce—W—Ca system, and a binder selected from kaolin, diatomaceous earth or cement is also added in order to maintain the strength of the catalyst.
Patent CN101992094A discloses a catalyst for dehydrogenation of ethylbenzene at low steam-to-oil ratio, wherein rubidium compound and one selected from the group consisting of Pm2O3, Eu2O3, Gd2O3 and Dy2O3 are added into a Fe—K—Ce—W—Ca system, and 0-4% binder selected from kaolin, diatomaceous earth or cement is also added in order to maintain the strength of the catalyst.
Patent CN102371161A discloses a catalyst for dehydrogenation of ethylbenzene at low steam-to-oil ratio, wherein cesium compound and one selected from the group consisting of Sm2O3, Eu2O3, Gd2O3 and Dy2O3 into a Fe—K—Ce—W—Mg system, and 0-4% binder selected from kaolin, diatomaceous earth or cement is also added in order to maintain the strength of the catalyst.
Patent CN103028421A discloses a catalyst for dehydrogenation of ethylbenzene at low steam-to-oil ratio, wherein potassium molybdate is added into a Fe—K—Ce—W—Mg system, and 2-5% binder selected from kaolin, diatomaceous earth or cement is also added in order to maintain the strength of the catalyst.
Patent CN103537696A discloses a catalyst for dehydrogenation of ethylbenzene at low steam-to-oil ratio, wherein manganese ferrite is added into a Fe—K—Ce—W—Mg system, and 0-4% binder selected from kaolin, diatomaceous earth or cement is also added in order to maintain the strength of the catalyst.
Patent CN103769150A discloses a catalyst for dehydrogenation of ethylbenzene at low steam-to-oil ratio, wherein potassium vanadate is added into a Fe—K—Ce—W—Mg system, and 2-5% binder selected from kaolin, diatomaceous earth or cement is also added in order to maintain the strength of the catalyst.
Catalysts in the published patents above use kaolin clay, diatomite or cement as a binder. Although the binder can improve the strength of the catalyst, the introduction of inert binder will partially cover the active site of the catalyst, resulting in decreased activity of the catalyst.
Patent CN1981929A discloses a low steam-to-oil ratio catalyst wherein, although no portland cement is added during the preparation of the catalyst, the strength of the catalyst is not disclosed, and the operation steam-to-oil ratio thereof is 1.8, which is high.